1. Field of the Invention
The present invention relates to .alpha.-olefin polymers in which the content of high-stereospecificity polymer expressed by 105.degree. C. xylene-insoluble fraction (XIS) is very low in spite of a low content of low-stereospecificity polymer expressed by 20.degree. C. xylene-soluble fraction (CXS), as well as to an .alpha.-olefin-polymerizing catalyst for obtaining such polymers, a process for producing such .alpha.-olefin polymers, and a polypropylene for use in the production of biaxially oriented films.
More particularly, the present invention relates to .alpha.-olefin polymers low in stickiness due to low CXS, excellent in stretching properties due to low XIS, and successfully usable for formation of films, fibers, blow molded articles, extrusion molded articles, etc., as well as to an .alpha.-olefin-polymerizing catalyst for obtaining such polymers, a process for producing .alpha.-olefin polymers, a polypropylene obtained by said process and suitable for use in the production of a biaxially oriented film excellent in stiffness and dimensional stability, and biaxially oriented films using said polypropylene.
2. Description of the Related Art
Because of their excellence in transparency, gloss, stiffness, water vapor barrier property, etc., biaxially oriented polypropylene films are extensively used as packaging materials. As the polypropylene used for production of biaxially oriented films, homopolymer of propylene is most conventional, and highly crystalline homopolymer of propylene obtained by the use of a high-stereospecificity catalyst has been used with the aim of improving the film properties, such as stiffness, of biaxially oriented film. However, such highly stereo-specific homopolymer of polypropylene is not good in the stretchability and has a fault that it is apt to make troubles such as film breaking in the process of stretching. Thus, a variety of methods for improving the stretchability of highly crystalline polypropylene have so far been proposed. As one example of such proposals, copolymerization of propylene with a small amount of ethylene is known.
Concretely speaking, JP-B-46-11027 has proposed a process for producing a polypropylene for use in production of films which comprises polymerizing propylene together with a very small amount of ethylene in the presence of a coordination catalyst and a hydro-carbon or chlorinated hydrocarbon solvent so that one mole of the monomer unit in the resulting polymer becomes containing 0.01 mole or less of ethylene unit. Further, in JP-B-64-6211, there is proposed a process for producing a polypropylene improved in stretching properties which comprises feeding propylene together with a small amount of ethylene into polymerization system using a catalyst consisting of an organoaluminum compound and titanium trichloride prepared by reducing titanium tetrachloride with an organoaluminum compound and further activating the reduced product by a treatment using a complex-forming agent, a treatment using an organoaluminum compound, a treatment using titanium tetrachloride or a combination thereof so that the resulting polymer comes to have an ethylene content of 0.1-1.0% by weight. Further, in JP-B-3-4371, there is proposed a process for producing a biaxially oriented polypropylene film excellent in transparency, stiffness and impact resistance by using a polypropylene of which ethylene content is 0.1-2% by mole and isotacticity is in a specified range.
Such processes, however, give no good result because copolymerization of only a small amount of ethylene causes a great deterioration in stereospecificity, a drop in stiffness and the troubles of stickiness, although stretching properties may be improved by these processes to a small extent.
As a method for improving the processing characteristics of the homopolymer, JP-A-61-23607 has proposed a method of using a composition prepared by mixing together two propylene homopolymers different in stereospecificity. Although processability may be somewhat improved by this method, the method requires to use a polypropylene mixture containing 90-10% by weight of a very low stereospecificity homopolymer in which the isotactic pentad fraction of boiling heptane-insoluble part is in the range of from 0.50 to 0.92, due to which the stiffness of polypropylene is sacrificed, the characteristic feature of the homopolymer itself cannot be exhibited and further the problem of stickiness arises.
As have been mentioned above, any of these prior methods are unsatisfactory from the viewpoint of obtaining a biaxially oriented polypropylene exhibiting excellent stiffness and dimensional stability simultaneously while maintaining a good stretchability.
The object of the present invention is to provide a novel .alpha.-olefin polymer capable of solving the above-mentioned problems of prior art, an .alpha.-olefin polymerizing catalyst for obtaining such a polymer and a process for producing said .alpha.-olefin polymer, and further provide a polypropylene for use in the production of biaxially oriented film capable of giving, when subjected to a stretching processing, a biaxially oriented film excellent in stiffness and dimensional stability, and a biaxially oriented film obtained from said polypropylene.